Optical see-through head-mounted displays (OST-HMD) find myriads of applications from scientific visualization to defense applications, from medical visualization to engineering processes, and from training to entertainment. In mixed or augmented reality systems, OST-HMDs have been one of the basic vehicles for combining computer-generated virtual scene with the views of a real-world scene. Typically through an optical combiner, an OST-HMD maintains a direct view of the physical world and optically superimposes computer-generated images onto the real scene. Compared with a video see-though approach where the real-world views are captured through cameras, it has the advantage of introducing minimal degradation to the real world scene. Therefore an OST-HMD is preferred for applications where a non-blocked real-world view is critical.
On the other hand, designing a wide field of view (FOV), low F-number, compact, and nonintrusive OST-HMD has been a great challenge, especially difficult for a non-pupil forming system. The typical eyepiece structure using rotationally symmetric components has limitations in achieving low F-number, large eye relief, and wide FOV. Many methods have been explored to achieve an HMD optical system which fulfils the above mentioned requirements. These methods include applying catadioptric techniques, introducing new elements such as aspherical surfaces, holographic and diffractive optical components, exploring new design principles such as using projection optics to replace an eyepiece or microscope type lens system in a conventional HMD design, and introducing tilt and decenter or even free-form surfaces. (H. Hoshi, et. al, “Off-axial HMD optical system consisting of aspherical surfaces without rotational symmetry,” SPIE Vol. 2653, 234 (1996). S. Yamazaki, et al., “Thin wide-field-of-view HMD with free-form-surface prism and applications,” Proc. SPIE, Vol. 3639, 453 (1999).)
Among the different methods mentioned above, free-form surfaces demonstrate great promise in designing compact HMD systems, In particular, a wedge-shaped free-form prism, introduced by Morishima et al. (Morishima et al., “The design of off-axial optical system consisting of aspherical mirrors without rotational symmetry,” 20th Optical Symposium, Extended Abstracts, 21, pp. 53-56 (1995)), takes the advantage of total internal reflection (TIR), which helps minimize light loss and improve the brightness and contrast of the displayed images when compared with designs using half mirrors. It is challenging, however, to design a free-form prism based OST-HMD offering a wide FOV, low F-number, and sufficient eye relief.
The concept of free-form HMD designs with a wedge-shaped prism was first presented by Morishima et al. in 1995, and the fabrication and evaluation method were explored by Inoguchi et al. (“Fabrication and evaluation of HMD optical system consisting of aspherical mirrors without rotation symmetry,” Japan Optics '95, Extended Abstracts, 20pB06, pp. 19-20, 1995). Following these pioneering efforts, many attempts have been made to design HMDs using free-form surfaces, particularly designs based on a wedge-shaped prism (U.S. Pat. Nos. 5,699,194, 5,701,202, 5,706,136. D. Cheng, et al., “Design of a lightweight and wide field-of-view HMD system with free form surface prism,” Infrared and Laser Engineering, Vol. 36, 3 (2007).). For instance, Hoshi et al. presented an FFS prism offering an FOV of 34° and a thickness of 15 mm; Yamazaki et al. described a 51° OST-HMD design consisting of a FFS prism and an auxiliary lens attached to the FFS prism; and more recently Cakmakci et al. designed a 20° HMD system with one free-form reflecting surface which was based on rational radial basis function and a diffractive lens. (“Optimal local shape description for rotationally non-symmetric optical surface design and analysis,” Opt. Express 16, 1583-1589 (2008)). There are also several commercially available HMD products based on the FFS prism concept. For instance, Olympus released their Eye-Trek series of HMDs based on free-form prisms. Emagin carried Z800 with the optical module WFOS, Daeyang carried i-Visor FX series (GEOMC module, A3 prism) products; Rockwell Collins announced the ProView SL40 using the prism technology of OEM display optics.
Existing FFS-based designs have an exit pupil diameter that is typically from 4 to 8 mm with a FOV typically around 40 degrees or less. In most of the existing designs, the size of the microdisplays is in the range of 1 to 1.3 inches, which affords a focal length of 35-45 mm for a typical 40-degree FOV. Even with an exit pupil up to 8 mm, the F/# remains fairly high (greater than 4) and eases the optical design challenge. A large size microdisplay, however, offsets the advantage of compactness using a free-form prism. In the more recent designs, smaller microdisplays, typically around 0.6″, were adopted, which requires a focal length of −21 mm to achieve a 40-degree FOV. The reduced focal length makes it very challenging to design a system with a large exit pupil. As a result, most of the designs compromise the exit pupil diameter. Thus, commercially available products on average reduce the pupil diameter to about 3-5 mm to maintain an F/# greater than 4. There are a few designs that achieve a larger pupil by introducing additional free-form elements or diffractive optical elements. For instance, Droessler and Fritz described the design of a high brightness see-through head-mounted system with an F/# as low as 1.7 by using two extra decentered lenses and applying one diffractive surface. (U.S. Pat. No. 6,147,807). The existing work shows that it is extremely difficult to achieve a very fast (low F/#) and wide field of view HMD design with a single wedge-shaped free-form surface prism.
Accordingly, it would be an advance in the field of optical see-through head-mounted displays to provide a head-mounted display which has a wide field of view and low F/#, while also providing a compact, light-weight, and nonintrusive form factor.